Active matrix electroluminescent display device

ABSTRACT

An active matrix electroluminescent display device in which the drive current through an electroluminescent display element ( 20 ) in each pixel ( 10 ) in a drive period is controlled by a driving device ( 22 ) based on a drive signal applied during a preceding address period and stored as a voltage on an associated storage capacitor ( 36 ). In order to counteract the effects of display element ageing, through which the light output for a given drive signal level diminishes over time, the pixel includes electro-optic discharging means ( 40 ) coupled to the storage capacitor and responsive to the display element&#39;s light output to leak stored charge and to control the integrated light output of the display element in the drive period. For improved control, the discharging means is arranged to rapidly discharge the capacitor at a controlled point in the drive period, upon the drive of the display element falling to a low level. A photoresponsive transistor can conveniently be utilised for this purpose.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 09/658,925, filed Sep. 11,2000, now U.S. Pat. No. 6,542,138.

This invention relates to active matrix electroluminescent displaydevices comprising an array of electroluminescent display pixels. Inparticular, the invention relates to an active matrix electroluminescentdisplay device comprising an array of display pixels each comprising anelectroluminescent display element and a driving device for controllingthe current through the display element in a drive period based on adrive signal applied to the pixel during an address period preceding thedrive period and stored as a charge on a storage capacitance associatedwith the driving device.

Matrix display devices employing electroluminescent, light-emitting,display elements are well known. The display elements may compriseorganic thin film electroluminescent elements, for example using polymermaterials, or else light emitting diodes (LEDs) using traditional III-Vsemiconductor compounds. Recent developments in organicelectroluminescent materials, particularly polymer materials, havedemonstrated their ability to be used practically for video displaydevices. These materials typically comprise one or more layers of anelectroluminescent material, for example a semiconducting conjugatedpolymer, sandwiched between a pair of electrodes, one of which istransparent and the other of which is of a material suitable forinjecting holes or electrons into the polymer layer. The polymermaterial can be fabricated using a CVD process, or simply by printing ora spin coating technique using a solution of a soluble conjugatedpolymer.

Organic electroluminescent materials exhibit diode-like I-V properties,so that they are capable of providing both a display function and aswitching function, and can therefore be used in passive type displays.

However, the invention is concerned with active matrix display devices,with each pixel comprising a display element and a switching device forcontrolling the current through the display element. Examples of anactive matrix electroluminescent (EL) display devices are described inEP-A-0653741 and EP-A-0717446. Unlike active matrix liquid crystaldisplay devices in which the display elements are capacitive andtherefore take virtually no current and allow a drive signal voltage tobe stored on the capacitance for the-whole frame period, the EL displayelements need to continuously pass current to generate light. A drivingdevice of a pixel, usually comprising a TFT, (thin film transistor), isresponsible for controlling the current through the display element. Thebrightness of the display element is dependent on the current flowingthrough it. During an address period for a pixel, a drive (data) signaldetermining the required output from the display element is applied tothe pixel and stored as a corresponding voltage on a storage capacitancewhich is coupled to, and controls the operation of, the currentcontrolling drive device with the stored voltage serving to maintainoperation of the switching device in supplying current through thedisplay element during a subsequent drive period, corresponding to aframe period, until the pixel is addressed again.

A problem with known organic electroluminescent materials, particularlypolymer materials, is that they exhibit poor stability and suffer ageingeffects whereby for example the light output for a given drive currentis reduced over a period of time of operation. While in certainapplications such ageing effects may not be critical, the consequencesin a pixellated display can be serious as any slight variations in lightoutput from pixels can easily be perceived by a viewer.

It is an object of the present invention to provide an active matrixelectroluminescent display device in which this problem is overcome atleast to an extent.

In the absence of developments in the electroluminescent materialsthemselves to improve their stability, it is believed that electronictechniques can be employed to provide appropriate electrical correctionfor the effects of such degradation.

According to the present invention there is provided an active matrixelectroluminescent display device comprising an array of display pixelseach of which comprises an electroluminescent display element and adriving device for controlling the current through the display elementin a drive period based on a drive signal applied to the pixel during anaddress period preceding the drive period and stored as a charge on astorage capacitance associated with the driving device, which ischaracterised in that each pixel includes electrooptic discharging meanscoupled to the storage capacitance for controlling the amount of lightoutput from the pixel in the drive period which discharging means isresponsive to light produced by the display element during the driveperiod and arranged to leak charge from the storage capacitance at arate dependent on the display element light output.

Thus, a given stored signal voltage for determining a desired lightoutput level of the display element of a pixel following addressing isprogressively changed in the drive period according to the light outputcharacteristic of the pixel's display element through operation of thedischarging means in the drive period, with the light output acting as afeedback variable, whereby the operation of the driving devicecontrolling energisation of the element (and hence light outputtherefrom) in the driving period is correspondingly progressivelyadjusted. The proportion of the available drive period for which thedisplay element is energised to produce light output is thereforedependent on, and regulated by, the action of the discharging means indischarging the storage capacitance according to its light output. Inthis way, the integrated light output from a display element in a frameperiod can be controlled so as to counteract the effects of ageing andimproved uniformity of display output is obtained even though thedegradation of individual display elements differs.

In order to obtain approximately a similar maximum amount of light froma degraded display element in the drive period, which correspondsapproximately to a frame period less the row address period, the amountof charge initially stored in the storage capacitance in the addressperiod may be increased slightly compared with that in the known displaydevice by increasing the magnitude of the data signal appropriately sothat a similar number of photons to that produced from an undegradeddisplay element can be obtained from the degraded display element beforethe value stored on the storage capacitance is reduced through operationof the discharging means, at a rate dependent on the light emission ofthe display element, until the driving device begins to turn off.Alternatively, the drive voltage applied to the display elements can beadjusted appropriately. Thus, the amount of light produced by thedegraded (aged) display element can be maintained similar to that fromthe display element before degradation.

The driving devices of the pixels preferably comprise TFTs and may beeither n type or p type TFTs, for example polysilicon MOS TFTs.References herein to discharging should therefore be construedappropriately in relation to the nature of the charge stored on thestorage capacitances in the address phase for both cases.

The discharging means preferably comprises a photoresponsive element inthe form of a photodiode which photodiode is connected to the storagecapacitance and arranged to be reverse biased in the drive period so asto leak charge from the storage capacitance in response to light fromthe display element falling thereon. Although it is envisaged that aphotoresponsive device other than a photodiode and operable in responseto light falling thereon to leak charge from the storage capacitance ata rate dependent on the level of the incident light in the drive periodmay alternatively be used, a photodiode is preferred for this purpose asits operation in leaking is independent of the voltage across it andsubstantially linearly proportional to incident light level.

With the driving device comprising a current-controlling transistor(TFT) connected in series with the display element between two supplylines at different voltage levels, and with the storage capacitancebeing connected between the gate node of the transistor and one of thesupply lines, as in the known device, then the photodiode can beconnected, with appropriate polarity to be reverse biased, in parallelwith the storage capacitance between the gate node and that supply line.Photocurrents generated in the photodiode by light from the displayelement result in leakage through the photodiode of the stored chargeand gradual reduction of the voltage at the gate node.

In a comparatively simple embodiment of the invention, the dischargingmeans may solely comprise a photodiode operating in the aforementionedmanner. Such an arrangement would be beneficial in overcoming to auseful extent problems due to display element degradation in manysituations.

However, if merely a photoresponsive element such as a reverse-biasedphotodiode connected across the storage capacitance is used as thedischarging means, problems may be experienced. As a result of theoperation of the discharging means in discharging the storagecapacitance, the level of light produced from the display elementgradually diminishes during the drive period and this will lead to thedisplay element being turned off in a slow manner which may not beadequately precise for the regulation control desired. At comparativelylow light output levels the operation of the pixel would becomeless-well controlled. Typically the characteristics of a photodiode aresuch that it becomes much less efficient at relatively low light levels.Also, in the case where the driving devices comprise TFTs, the operationof an individual driving device becomes less well defined as the gatevoltage begins to approach its threshold voltage and so non-uniformoperation of the devices of the array could occur.

In a preferred embodiment of the invention, therefore, the dischargingmeans is further arranged to rapidly discharge the storage capacitanceand curtail light output from the display element at a point in thedrive period which is controlled by, and dependent on, the operation ofthe display element. The operation of the discharging means in thisrespect is preferably determined by an operational characteristicindicative of the light output of the display element dropping to acertain, lower, level. Consequently, problems due to the nature of thebehaviour of a photo-responsive element such as a photodiode at lowlight levels and the operation of the driving TFT close to its thresholdvoltage are avoided. The discharging means could be made responsivedirectly to the light output of the display element for this purposebut, preferably, the operation of the discharging means in this respectis made dependent on an electrical parameter that varies in accordancewith the drive level of the display element, for example according tothe level of electrical current flowing through the display element or avoltage in the pixel circuit which varies in accordance with suchcurrent (the light level of the display element being dependent on thiscurrent). The controlled point is then determined by the electricalparameter reaching a predetermined level. This enables electrically,rather than optically, responsive switching devices or circuits to beused for this purpose.

Because the light output from the display element is suddenly terminatedrather than diminishing very gradually, grey scales are made easier tocontrol and efficiency is improved.

In this embodiment, the discharging means may comprise a photoresponsiveelement, again preferably a reverse-biased photodiode, connected acrossthe storage capacitance and responsive to light generated by the displayelement to leak away charge stored on the storage capacitance slowly,and a switching device, such as a transistor, which is connected inparallel with the photoresponsive element across the storage capacitanceand responsive to current flow through the display element to dischargethe storage capacitance rapidly upon the level of current flow throughthe display element reaching a certain, low, level.

Preferably, however, the discharging means comprises a photoresponsivetransistor connected across the storage capacitance through which chargeis leaked away by photocurrents generated therein by light from thedisplay element and whose gate is coupled to a source of potentialdependent on the current flowing through the display element. Onecurrent carrying electrode of the transistor, for example the drainjunction, can be arranged reverse biased and responsive to light fallingthereon so that the drain-source path behaves as a reverse-biasedphotodiode while the transistor is off and with the potential applied toits gate controlling its switching operation. Thus, only one device isneeded to fulfil the required functions for the discharging means of,initially, slow and, subsequently rapid discharge of the capacitance.Such a transistor is relatively simple and convenient to fabricatealongside, and simultaneously with, the transistors of the active matrixcircuit, e.g. the driving TFTs.

Conveniently, the gate of the photo-responsive transistor may be coupledto the node between the display element and the driving device. Thevoltage at this node varies according to the level of current flowthrough the display element. As the storage capacitance discharges dueto photocurrent in the photo-responsive transistor, the gate voltagedecreases and the voltage across the display element increases until ata certain point the photoresponsive transistor's threshold level isreached causing it to turn on and rapidly discharge the capacitance.

When using a TFT as the driving device of a pixel, the invention offersa further important advantage. Since the drive current for a displayelement is determined by the voltage applied to the gate of the TFT,corresponding to the voltage stored in the capacitance, this drivecurrent depends strongly on the characteristics of the TFT and so anyvariations in the threshold voltage, mobility and dimensions of theindividual TFTs of pixels over the array, for example due tomanufacturing process tolerances, can produce unwanted variations in thedisplay element currents and hence output light levels produced, whichcauses non-uniformities in the display output. The effect of thedischarging means in controlling the stored voltage signal will alsocompensate to an extent for such variations in TFT characteristics.

A further advantage is that the operation of the discharging means canalso negate at least to some extent problems due to voltage dropsoccurring during the drive period in a common current line connected to,and shared by, the display elements of, for example, all the pixels of arow.

Although the invention is particularly beneficial in devices usingpolymer LED materials, it can of course be applied to advantage in anyactive matrix electroluminescent device in which the electroluminescentmaterial used similarly suffers ageing effects resulting in lower lightoutput levels for a given drive current over a period of time ofoperation.

Embodiments of active matrix electroluminescent display devices inaccordance with the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:—

FIG. 1 is a simplified schematic diagram of known active matrixelectroluminescent display device comprising an array of pixels;

FIG. 2 shows the equivalent circuit of a few typical pixels of the knownactive matrix electroluminescent display device of FIG. 1;

FIG. 3 shows the equivalent circuit of one typical pixel in a firstembodiment of display device according to the present invention;

FIG. 4 shows several pixels of the device of FIG. 3 and an example ofthe manner of their connection;

FIG. 5 shows the equivalent circuit of a few typical pixels in a secondembodiment of active matrix electroluminescent display device accordingto the invention;

FIGS. 6 and 7 are graphs illustrating the operation of a representativepixel in the device of FIG. 5; and

FIG. 8 shows the equivalent circuit of an alternative form of pixel inanother embodiment according to the invention.

The Figures are merely schematic. The same reference numbers are usedthroughout the Figures to denote the same or similar parts.

Referring to FIG. 1, the active matrix electroluminescent display devicecomprises a panel having a row and column matrix array ofregularly-spaced pixels, denoted by the blocks 10, each comprising anelectroluminescent display element and an associated driving devicecontrolling the current through the display element, and which arelocated at the intersections between crossing sets of row (selection)and column (data) address conductors, or lines, 12 and 14. Only a fewpixels are shown for simplicity. The pixels 10 are addressed via thesets of address conductors by a peripheral drive circuit comprising arow, scanning, driver circuit 16 and a column, data, driver circuit 18connected to the ends of the respective sets.

Each row of pixels is addressed in turn by means of a selection signalapplied by the circuit 16 to the relevant row conductor 12 so as to loadthe pixels of the row with respective drive signals, determining theirindividual display outputs, according to the respective data signalssupplied in parallel by the circuit 18 to the column conductors. As eachrow is addressed, the data signals are supplied by the circuit 18 inappropriate synchronisation.

FIG. 2 illustrates the circuit of a few, typical, pixels in this knowndevice. Each pixel, 10, includes a light emitting organicelectroluminescent display element 20, represented here as a diodeelement (LED), and comprising a pair of electrodes between which one ormore active layers of organic electroluminescent material is sandwiched.In this particular embodiment the material comprises a polymer LEDmaterial, although other organic electroluminescent materials, such aslow molecular weight materials, could be used. The display elements ofthe array are carried together with the associated active matrixcircuitry on one side of an insulating support. Either the cathodes orthe anodes of the display elements are formed of transparent conductivematerial. The support is of transparent material such as glass and theelectrodes of the individual display elements 20 closest to thesubstrate can consist of a transparent conductive material such as ITOso that light generated by the electroluminescent layer is transmittedthrough these electrodes and the support so as to be visible to a viewerat the other side of the support. Alternatively, the light output couldbe viewed from above the panel and the display element anodes in thiscase would comprise parts of a continuous ITO layer constituting asupply line common to all display elements in the array. The cathodes ofthe display elements comprise a metal having a low work-function such ascalcium or magnesium silver alloy. Examples of suitable organicconjugated polymer materials which can be used are described in WO96/36959. Examples of other, low molecular weight, organic materials aredescribed in EP-A-0717446, which also describes examples of theconstruction and operation of an active matrix electroluminescent deviceand whose disclosure in these respects is incorporated herein byreference.

Each pixel 10 includes a driving device in the form of a TFT 22 whichcontrols the current through, and hence operation of, the displayelement 20 based on a data signal voltage applied to the pixel. Thesignal voltage for a pixel is supplied via a column conductor 14 whichis shared between a respective column of pixels. The column conductor 14is coupled to the gate of the current-controlling drive transistor 22through an address TFT 26. The gates for the address TFTs 26 of a row ofpixels are all connected to a common row conductor 12.

Each row of pixels 10 also shares a common voltage supply line 30, heldat a predetermined potential and usually provided as a continuouselectrode common to all pixels, and a respective common current line 32.The display element 20 and the driving device 22 are connected in seriesbetween the positive voltage supply line 30 and the common current line32, which is at a negative potential with respect to the supply line 30,for example ground, and acts as a current source for the current flowingthrough the display element 20. The current flowing through the displayelement 20 is controlled by the switching device 22 and is a function ofthe gate voltage on the transistor 22, which is dependent upon a storedcontrol signal determined by the data signal supplied to the columnconductor 14.

A row of pixels is selected and addressed by the row driver circuit 16applying a selection pulse to the row conductor 12 which switches on theaddress TFTs 26 for the respective row of pixels and define a respectiverow address period. A voltage level derived from the video informationsupplied to the driver circuit 18 is applied to the column conductor 14by the driver circuit 18 and is transferred by the address TFT 26 to thegate node 24 of the drive transistor 22. During the periods when a rowof pixels is not being addressed via the row conductor 12 the addresstransistor 26 is turned off, but the voltage on the gate node of thedrive transistor 22 is maintained by a pixel storage capacitor 36 whichis connected between the gate of the drive transistor 22 and the commoncurrent line 32, so as to maintain the operation of the display elementduring this drive period. The voltage between the gate of the drivetransistor 22 and the common current line 32 determines the currentpassing through the display element 20 of the pixel 10. Thus, thecurrent flowing through the display element is a function of thegate-source voltage of the drive transistor 22 (the source of then-channel type transistor 22 being connected to the common current line32, and the drain of the transistor 22 being connected to the displayelement 20). This current in turn controls the light output level(grey-scale) of the pixel.

The switching transistor 22 is arranged to operate in saturation, sothat the current flowing through the transistor is insensitive to thedrain-source voltage and dependent on the gate-source voltage.Consequently, slight variations of the drain voltage do not affect thecurrent flowing through the display element 20. The voltage on thevoltage supply line 30 is therefore not critical to the correctoperation of the pixels.

Each row of pixels is addressed in turn in this manner in a respectiverow address period so as to load the pixels of each row in sequence withtheir drive signals and set the pixels to provide desired displayoutputs for the drive period, corresponding approximately to a frameperiod, until they are next addressed.

With this known pixel circuit, it will be appreciated that the voltagestored on the capacitor 36 is substantially determined by the applieddata signal voltage and that as this voltage in turn controls the drivetransistor 22 and the current through the display element 20 then theresulting light output level of the display element at any time will bedependent on the then existing current/light output level characteristicof the display element and is substantially constant throughout theframe period. The electroluminescent material of the display element cansuffer degradation over a period time of operation leading to ageingeffects. Ageing of an element will change the voltage drop across theelement as well as the efficiency of its operation in terms of the lightoutput provided for a particular current drive level. Either or both ofthese effects can contribute to the degradation problem. Those pixelswhich have, therefore, been driven longer (or harder) will exhibitreduced brightness and cause display non-uniformities. With polymer LEDmaterials the effects of such ageing can be significant.

In the present invention, an opto-electronic technique is utilised ineach pixel for effectively responding to display element degradation andcounteracting this effect by controlling the integrated light output ina frame period accordingly. In this technique optical feedback is usedin a manner such as to discharge the storage capacitor at a ratedependent on the instantaneous light emission of the display elementduring the driving period. Consequently, for a given data signal thelength of time for which a display element is energised to generatelight during the drive period following the address period is regulatedaccording to the subsisting light emission characteristic of the displayelement, as well as the level of the applied data signal, such that thelight output from the pixel is substantially the same as would beobtained with a non-degraded display element. If a higher brightness isrequired from one pixel, the display element is energised longer (up toa maximum corresponding to the available drive, frame, period) byincreasing the applied data signal and vice versa.

Referring to FIG. 3, there is shown schematically the equivalent circuitof one typical pixel in a first embodiment of display device accordingto the invention intended to overcome, at least to an extent, the effectof ageing using this technique. In this pixel 10 the display element 20is similarly connected in series with the n-channel drive transistor 22(again operating in saturation mode) between the current line 32, commonto a row of pixels and for example at ground potential, and a voltagesupply line 30, at a positive potential with respect to the line 32, andthe gate node 24 of the TFT 22 is connected to the drain of the addresstransistor 26 whose gate and source terminals are supplied respectivelywith selection and data signals via the associated row and columnconductors 12 and 14. The storage capacitor 36 is again connectedbetween the gate node of the drive transistor 22 and the current line32.

The pixel 10 further includes electro-optic discharging means fordischarging the storage capacitor 36 which comprises a photodiode 38connected in parallel with the storage capacitor 36 between the gatenode 24 of the TFT 22 and the current line 32 and arranged inreverse-biased manner with its cathode and anode terminals connectedrespectively to the gate node and the line 32. The photodiode 38, forexample formed from amorphous silicon and having a pin structure, isarranged physically in relation to the display element 20 so as toreceive part of the light generated by the element 20 in operation ofthe device as indicated by the wavy arrows in FIG. 3. The photodiode maybe shielded from ambient light. Like the known pixel circuit, operationof this pixel circuit has two phases, an addressing phase in which thepixel is set to a desired display output condition dependent on anapplied data signal and a subsequent drive phase in which the displayelement is driven according to the set condition until the pixel isagain addressed, for example in the next frame.

In the addressing phase, the pixel is addressed in similar manner tothat previously described with a selection pulse signal being applied tothe row conductor 12 so as to turn on the address TFT 26 in a respectiverow address period and charge the gate node of the TFT 22 to a drivesignal voltage value according to the level of a data signal present onthe column conductor 14. This stored value is responsible for operatingthe TFT 22 and determining the drive current through the display elementin the drive period following the turning off of the address TFT 26 atthe end of the row selection, address, period.

In the drive period, the optical coupling between the display element 22and the reverse biased photodiode 38 means that-light emitted by thedisplay element and incident on the photodiode generates a smallphotocurrent therein which is approximately linearly proportional to theinstantaneous light output level. As a consequence of this photocurrent,the photodiode 38 leaks charge on the gate node 24 and the nodegradually discharges through the photodiode until the gate voltage ofthe TFT 22 reaches its threshold level whereupon the TFT 22 turns offand current flow through the display element 20 ceases. Such dischargeoccurs at a rate dependent on the instantaneous light emission level ofthe display element.

This optical feedback technique enables the total, integrated, amount oflight emitted by the display element within the drive period, whichviewer perceives as brightness, to be regulated and allows the effectsof display element degradation to be counteracted. The integrated lightoutput (brightness) is dependent on the length of time in the driveperiod for which the display element is energised as well as its initiallight level. Because of the action of the discharging means indischarging the storage capacitor at a rate dependent on the actuallight output level of the display element, and thus in controlling theduration for which the display element is energised in the drive period,then different pixels in the array supplied with the same data signalvalue will tend to produce similar perceived brightness levelsregardless of variations in the degradation of individual displayelements. In other words, the integral of the light outputs fromindividual display elements addressed with the same data signal valuewill be similar even though at the start of the drive period theirrespective light output levels may differ due to degradation effects.The optical feedback effectively compensates for such differences byregulating the duration of energisation of individual pixels inaccordance with their light output so a more uniform display output isobtainable.

As usual, the level of the applied data signal is adjusted appropriatelyto provide different grey-scale levels from the pixels. If the datasignal, and thus charge on the gate node 24, is increased then morephotons are required from the display element during the drive periodbefore the TFT 22 is caused to switch off, so that a higher grey-scalelevel is achieved, and vice versa.

In order to obtain a maximum level of brightness-from degraded displayelements similar to that achieved with undergraded display elements in aconventional display device, then the amplitude range for the applieddata signal voltage may be correspondingly increased or, alternatively,the drive voltage level for the display elements can be adjustedappropriately, for example by increasing the voltage level of the line30.

FIG. 4 illustrates a few, typical, pixels using this circuit in an arrayand an example of their interconnection with common supply lines andassociated row and column address lines. In this particular scheme, arespective current supply line 32 is shared by pixels in two adjacentrows. The common lines 30 may in practice be constituted by a continuousconductor layer common to all pixels. Yn, Yn+1, Yn+2 etc., are rowaddress lines, one for each row of pixels, to which selection signals(Vs) are applied in turn to address each row of pixels in sequence, andXn, Xn+1, Xn+2 etc., are column address lines via which the data signalsare supplied to the associated pixels in a selected row.

Referring to FIG. 5, there is shown the equivalent circuit of a fewtypical pixels in a second embodiment of display device according to theinvention employing an electro-optical feedback circuit for overcoming,at least to an extent, the effects of ageing. In each pixel 10 thedisplay element 20 is similarly connected in series with the drivetransistor 22 (again operating in saturation mode) between the commoncurrent line 32, for example at ground potential, and the voltage supplyline 30, at a positive potential with respect to the line 32, here shownconstituted by a common anode electrode layer shared by all the pixels,and the gate and source of the address transistor 26 are connected tothe associated row and column conductors 12 and 14 respectively. Asbefore, the storage capacitor 36 is connected between the gate node 24of the drive transistor 22 and the current line 32.

The pixel 10 further includes electro-optic discharging means comprisinga photo-responsive device 40 which here comprises another TFT whosesource and drain electrodes are connected across the storage capacitor36, to the gate node 24 of the drive transistor 22 and the current line32, and whose gate is connected to the node, 41, between the drivetransistor 22 and the display element 20. In the case when the drivetransistor 22 comprises an n-type low temperature polysilicon TFT, thedevice 40 is of a similar type.

The pixel is constructed and arranged in such a way that thephotoresponsive device 40 is exposed to some of the light emitted by thedisplay element 20 in operation of the pixel. The consequence of suchoptical coupling between these components will become apparent from thefollowing description of the pixel's operation.

As with the known pixel circuit and the previous embodiment, operationof this pixel circuit has two phases, an addressing phase in which thepixel is set to a desired display output condition dependent on anapplied data signal and a subsequent drive phase in which the displayelement is driven according to the set condition until the pixel isagain addressed, for example in the next frame. In the addressing phase,the row driver circuit 16 applies a selection pulse signal to the rowconductor 12 in a respective row address period which turns on the TFT26 of each pixel in the row, and respective data voltage signals areapplied to the column conductors 14 by the driver 18. The capacitor 36is assumed for simplicity here to be fully discharged at the start ofthe addressing period. As a consequence, a voltage is set on the gatenode 24 of the drive TFT 22 according to the level of the data signaland the capacitor 36 is charged to this voltage level, which, afterremoval of the selection pulse at the end of the row address period,serves to maintain the gate voltage of the TFT 22 at least initially inthe subsequent drive phase as in the known pixel.

The drain junction of the photo-responsive TFT 40, which is coupled tothe gate node of the TFT 22, is reverse biased. This junction isphotosensitive and, as a result of the device 40 being optically coupledwith the display element 20 light emitted by the display element in thedrive period falling on this device, causes a small photocurrent to beproduced in the device 40 which, because it is reverse biased, isapproximately linearly proportional to the display element'sinstantaneous light output level. The effect of this photocurrent is toslowly discharge the storage capacitor 36, the amount of photocurrent,and thus the rate of discharge, being dependent on the light outputlevel of the display element. At this initial stage of the drive period,the voltage on the gate of the device 40, corresponding to the voltageat the node 41, is relatively small and below the threshold voltagelevel of the transistor 40. Accordingly, because of the photocurrentsgenerated therein, the transistor 40 behaves at this time merely as aleakage device, in the manner of a reverse biased photodiode, leading tocharge on the capacitor 36 leaking away therethrough.

Consequently, the capacitor 36 slowly discharges in the drive period andthe voltage on the gate of the drive TFT 22 gradually reduces to lowerthe current flowing through the display element 20 as the TFT 22approaches its threshold, turn-off, level. The light output of thedisplay element decreases in corresponding fashion with the reduction ofthe gate voltage. The reduction in current flowing through the displayelement 20 leads to a gradual increase in the voltage level at the node41 and at a certain point in time, corresponding to the light outputlevel attaining a predetermined lower limit, the voltage at the node 41relative to the line 32 reaches the threshold voltage level of thetransistor 40 causing the transistor 40 to abruptly turn on. The effectof this is to discharge very rapidly the capacitor 36, and to bring thegate voltage of the TFT 22 to the potential level of the line 32,thereby quickly turning off the TFT 22 and preventing any furthercurrent flow through the display element 20 so that light output fromthe pixel suddenly ceases. Thus, the light output is terminated at adefined stage during the drive period when the level of this lightoutput falls to a particular level. As examples of typical voltagespresent in operation of the pixel, assuming for example that both TFTs22 and 40 have a 5 volt threshold, the voltage supply line 30 may be ataround 8 volts, the common current line 32 may be at 0 volts, and as thevoltage at the gate node of the transistor 22 changes from 10 to 6 voltsthe voltage at the node 41 can change from 2 volts to 5 volts.

The operation of the pixel in this respect is illustrated graphically inFIG. 6 which shows the relationship between the display element'sinstantaneous light output level, I, (i.e. the number of photons emittedper second) and time, T. On the time axis, Td denotes the start of thepixel's drive period, immediately following its address period, and Tfrepresents the end of the frame period. At the beginning of the driveperiod, therefore, the light output from the display element isrelatively high (as determined by the applied data signal). Photocurrentgenerated in the device 40 then has the effect of gradually, andgenerally linearly, reducing the gate voltage of the TFT 22, so that thedisplay element's light output level decreases in corresponding fashion,as shown in the region X in FIG. 4. Upon the gate threshold voltagelevel of the device 40 being attained at the node 41, as a result of thedrive current through the display element falling to a certain lowerlimit, the device 40 switches on, at point Ty, whereupon the TFT 22rapidly turns off and light output from the element 20 ceases.

The dotted portion of the curve shown in FIG. 6 illustrates the lightoutput effect which could be obtained if simply a photo-responsivedevice such as a reverse-biased photodiode as in the first embodimentwere to be connected across the capacitor 36 instead of the device 40.As can be seen, there would then be a much slower switch off of thecurrent through the display element. Due to the photon output of thedisplay element gradually decreasing to a comparatively low-level andthe typical response behaviour of a photodiode, a long “tail” in thecurve will likely result as shown. Such a tail makes control at lowlight levels more difficult as, firstly, the drive TFT 22 is operatingclose to its threshold voltage level at this stage and, secondly, lightemission is occurring in the least efficient part of the photodiodecharacteristic. The pixel circuit in this second embodiment overcomessuch problems with grey-scales being easier to achieve and efficiencyimproved.

The length of time over which the display element is energised in partdetermines the total amount of light output, and hence brightness (theinitial current drive level also being a factor) and this period canvary up to a maximum corresponding to the frame period (Tf) at whichtime the pixel is again addressed. The light which is output by thedisplay element during the drive period is integrated by the eye of aviewer and hence the amount of light, brightness, perceived by theviewer effectively is proportional to the area below the curve. Throughthe operation of the discharging means, this area, i.e. integral of thelight output, can be made to remain substantially the same for a givendata signal regardless of degradation in individual display elements.

In order to vary the light output from a display element in a frameperiod, and thus its grey-scale, the charge placed on the gate node ofthe TFT 22 during the addressing period is adjusted appropriately byincreasing or decreasing the data signal voltage level. If the charge isincreased, more photons are required from the display element during thedischarging period before the TFT 22 switches off and vice versa.

Because the light output of the display element diminishes over theframe, then to obtain a similar maximum level of brightness to thatachieved by an undegraded pixel in a conventional device in which thelight input level remains substantially constant for the frame period,such brightness corresponding to the area below the curve in FIG. 6,then the maximum value of the applied data signal can be increased sothat the initial illumination level (at Td) is increased. Alternatively,and perhaps preferably, the level of the drive voltage applied to thedisplay element can be adjusted appropriately, for example by raisingthe voltage level of the line 30.

FIG. 7 is similar to FIG. 6 but graphically illustrates the operation ofa pixel at different drive levels and the effects of ageing. In thisgraph, curves I and II represent light output for two different levelsof applied data signal at an early stage in the device's operating life,the level for I being higher than the level for II. It is clearly seenthat the areas bounded by these two curves are substantially different.The curves III and IV show the effect of ageing and degradation in thedisplay element at a later stage in the device's life for similar datasignal levels used for the obtaining the curves I and II respectively.Comparing curves I and III, therefore, it is seen that the light outputlevel initially and throughout the drive period is reduced through theeffects of ageing but that, as a result of the operation of thedischarging means, the total amount of light produced, as determined bythe area beneath the curve, is maintained by the period of energisationbeing varied (increased) appropriately. Curves II and IV show a similarresult for a lower light output level.

The discharging means comprising the photo-responsive transistor 40 doesnot have any appreciable effect during the addressing period. At thebeginning of the next addressing period, the transistor 40 is still on.However, the voltage drop across this transistor means that the gatenode of the transistor 22 will rise upon the application of the new datasignal and transistor 22 will start to conduct, thereby lowering thevoltage at the node 41 and causing the transistor 40 to turn off. Asmall resistor can be connected between the drain of the transistor 40and the junction between the capacitor 36 and the gate of the transistor22 if required to ensure such operation. Because the addressing periodis significantly shorter than the drive period, for example less than 32microseconds with a frame period of around 20 milliseconds, the effectsof the leakage through the transistor 40 thereafter due to photocurrentsduring the addressing period are not significant.

The photo-responsive transistor 40 is entirely compatible with the driveTFT 22 (both can be formed as low temperature polysilicon TFTs of thesame type, e.g. n type), and typical levels of photocurrents generatedin the transistor 40 in operation are compatible with an easilyintegrated storage capacitor.

Besides compensating automatically for display element ageing effects,the manner of operation of the pixel circuit (and likewise that of thefirst embodiment) means also that it is effective to compensateautomatically for variations in the operational characteristics of theTFTs 22 of different pixels in the array resulting, for example, fromvariations in their threshold voltages, dimensions, and mobilities dueto the nature of the thin film fabrication processes used to form theTFTs. As a result, further improvement in the uniformity of light outputfrom the display elements over the array is achieved. In addition, thepixel circuit assists in avoiding unwanted effects caused in the driveperiod by voltage drops occurring in the common current lines 32 and 30.

The TFTs used in the above described embodiments of pixel circuits allcomprise n-channel MOS TFTs. However, p-channel TFTs could be usedinstead, with the polarity of the display element 20, and the applieddrive voltages being reversed. In this case, references to dischargingused herein should be construed accordingly, as will be apparent toskilled persons. Preferably, polysilicon TFTs are used, althoughalternatively amorphous silicon TFTs could be employed.

Although the transistor 40 is arranged so that it is exposed to lightemitted by the display element 20, it is preferably shielded fromambient light falling on the device so that it is responsive inoperation solely to light from the display element.

The current lines in the above embodiment may instead extend in thecolumn direction with each current line then being shared by arespective column of pixels.

Although in this second embodiment a photoresponsive transistor ispreferred for the opto-electronic discharging means, it is envisagedthat the discharging means could be of a different form, for examplecomprising a switching device such as a TFT and a separate photodiodeconnected in parallel across the capacitor 36 with similarly the gate ofthe TFT being connected to the node 41 and with the photodiode beingreverse-biased. It is envisaged also that operation of the switchingdevice of the discharging means may be controlled other than byconnecting its gate to the node 41, for example by connecting its gateto some other point in the pixel circuit whose voltage varies inaccordance with drive current flowing through the display element suchthat the switching device is turned on in response to the drive currentfalling to a certain lower level.

The operation of the discharging means in switching to rapidly dischargethe storage capacitor may alternatively be controlled by a current inthe pixel circuit which varies in accordance with current flow throughthe display element or by means of an optically sensitive switch circuitwhich is responsive directly to the light emitted by the display elementand which has a threshold characteristic so as to switch to dischargethe capacitor upon the level of the light received by the circuitdropping to a predetermined value.

The invention can be used also with pixels driven using a current datasignal rather than a voltage data signal as in the above-describedembodiment. FIG. 8 illustrates another form of pixel circuit suitable tobe used with current data signals. The same reference numbers are usedto designate corresponding components. Apart from the discharging means40, this pixel circuit is similar to an example described in WO99/65012to which reference is invited for a detailed description. Briefly, thepixel circuit includes additionally two further TFTs 50 and 51interconnected between the gate node of the TFT 22, the line 32 and theoutput of the address TFT 26 as shown. The further TFT 50 and the TFT 22form a current mirror circuit whose operation provides in the pixelscompensation for variation in the threshold voltage of the TFTs 22.

In a row address period the TFTs 26 and 51 are turned on and the TFT 50samples an input, data, current flowing in the conductor 14 via the TFT26. This current is mirrored by the TFT 22 to produce a proportionalcurrent through the display element 20. Once the current has stabilised,the voltage across the storage capacitor 36 becomes equal to the gatevoltage on the TFTs 22 and 50 required to produce this current.

The discharging means 40 behaves in the subsequent drive period as inthe previous embodiment.

Thus, in summary, an active matrix electroluminescent display device hasbeen described in which the drive current through an electroluminescentdisplay element in each pixel in a drive period is controlled by adriving device based on a drive signal applied during a precedingaddress period and stored as a voltage on an associated storagecapacitor. In order to counteract the effects of display element ageing,through which the light output for a given drive signal level diminishesover time, the pixel includes electro-optic discharging means coupled tothe storage capacitor and responsive to the display element's lightoutput to leak stored charge and to control the integrated light outputof the display element in the drive period. For improved control, thedischarging means is arranged to rapidly discharge the capacitor at acontrolled point in the drive period, upon the drive of the displayelement falling to a low level. A photoresponsive transistor canconveniently be utilised for this purpose.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the field of active matrixelectroluminescent display devices and component parts therefor andwhich may be used instead of or in addition to features alreadydescribed herein.

What is claimed is:
 1. An active matrix electroluminescent displaydevice, comprising: an array of display pixels, each display pixelincluding an electroluminescent display element and a driving device forcontrolling the current through said electroluminescent display elementin a drive period based on a drive signal applied to saidelectroluminescent display pixel during an address period preceding thedrive period and stored as a charge on a storage capacitance associatedwith said driving device, and electro-optic discharging means coupled tothe storage capacitance for controlling the amount of light output fromsaid display pixel in the drive period, said electro-optic dischargingmeans being responsive to light produced by said electroluminescentdisplay element during the drive period and arranged to leak charge fromthe storage capacitance at a rate dependent on an output level of lightof said electroluminescent display element in the drive period, whereinsaid electro-optic discharging means rapidly discharges the storagecapacitance and curtails light output from said electroluminescentdisplay element at a point in the drive period which is controlled by,and dependent on, an operation of said electroluminescent displayelement, and wherein a controlled point at which said electro-opticdischarging means operates to rapidly discharge the storage capacitanceis determined by an operational characteristic of said display pixelindicative of the light output of said electroluminescent displayelement reaching a certain low level.
 2. The active matrixelectroluminescent display device of claim 1, wherein the controlledpoint at which said electro-optic discharging means operates to rapidlydischarge the storage capacitance is determined by an electricalparameter that varies in accordance with the drive level of saidelectroluminescent display element.
 3. The active matrixelectroluminescent display device of claim 2, wherein the operation ofsaid electro-optic discharging means to rapidly discharge the storagecapacitance is controlled according to the level of electrical currentflowing through said electroluminescent display element.
 4. The activematrix electroluminescent display device of claim 1, wherein saidelectro-optic discharging means includes a switching device connectedacross the storage capacitance and operable to rapidly discharge thestorage capacitance.
 5. The active matrix electroluminescent displaydevice of claim 4, wherein said driving device includes a transistorconnected in series with said electroluminescent display element, andwherein an operation of said switching device is controlled by a voltageacross said driving device.
 6. The active matrix electroluminescentdisplay device of claim 4, wherein said driving device and saidswitching device include TFTs.
 7. The active matrix electroluminescentdisplay device of claim 6, wherein said electro-optic discharging meansincludes a photo-responsive element connected across the storagecapacitance and responsive to light output from said electroluminescentdisplay element to leak charge from the storage capacitance.
 8. Theactive matrix electroluminescent display device of claim 7 wherein saidphotoresponsive element includes a reverse biased photodiode.
 9. Theactive matrix electroluminescent display device of claim 4, wherein saidelectro-optic discharging means includes a photo-responsive transistorthrough which charge on the storage capacitance is leaked byphotocurrent generated therein by light from said electroluminescentdisplay element, and wherein a gate of the photo-responsive transistoris coupled to a source of potential dependent on current flow throughsaid electroluminescent display element.
 10. The active matrixelectroluminescent display device of claim 9, wherein said gate of saidphoto-responsive transistor is further coupled to a node between saidelectroluminescent display element and said driving device.